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Exhaust gas analyser - what carbon monoxide level should I expect from my BT7?

fishyboy

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Hi all,
I am thinking of buying an exhaust gas analyzer (Gunsons, https://www.gunson.co.uk/product/G4125) to help with getting the mixture right on my BT7 twin carb. I currently use two Gunson Colourtune's ( in cylinder no. 2 and 5) but always feel the mixture is too rich despite getting a blue colored combustion.

While I understand the I could use the exhaust gas analyzer to match the CO output from the two tail pipes (assuming they are each fed from separate carbs I am less clear about what the ideal CO % should be. I have read anywhere from 1-6%?

Does anyone have any experience of using a Gunson's exhaust gas analyzer? Also does anyone know what the optimized CO level should be for a BT7?

thanks

Phil
 

Keoke

Great Pumpkin
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Phil Says:
"I am less clear about what the ideal CO % should be."

The CO% level that passes the Bloody MOT! :devilgrin:
 

Gearhead_Garage

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I would get a dual-channel wideband sensor and gauge. Put an O2 sensor in each downpipe and you can constantly monitor the air/fuel mixture of each carb at all places in the rev range. The ideal lambda ratio is between 13.7 and 14.5.

I have an AEM wideband system in the race cars and an APSX system in my vintage BMW. So far I have not taken my own advice on the Healey.
 
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fishyboy

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Keoke
The UK MOT only requires a visual inspection of the exhaust gases for pre-1975 engines. No CO% is measured.

Gearhead
The device I'm looking to buy only measures CO%

Searching around the web I found the attached table of air-fuel mixture (AFM)v CO%. Ideal combustion is at AFR 14.7 which equates to 0.1% CO. Most folk appear to settle on a CO 1-4%.

Phil


CO% v AF.jpg
 

Gearhead_Garage

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Searching around the web I found the attached table of air-fuel mixture (AFM)v CO%. Ideal combustion is at AFR 14.7 which equates to 0.1% CO.
14.7 is ideal but you always want to be just a bit on the rich side to prevent detonation so optimal Air/Fuel readings would be 13.7 - 14.5.
 

vette

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from a very practical, non-engineering point of view I would say that for a car such as a Healey that the readings are going to be relative. Meaning the best you can get. I'm guessing that with a stock engine, including intake, carbs, exhaust you won't get near an ideal set up. To have good driveability and no pinging it is going to be on the rich side. Once you come up with a setting that works then that would be your mark to tune by from then on. I've thought about buying some type of analyzer to tune with, it would be a great tool. But ya know, the lift the piston method on the carbs does the same thing. Setting mixture with a tach is also very effective.
 
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... Does anyone have any experience of using a Gunson's exhaust gas analyzer? Also does anyone know what the optimized CO level should be for a BT7?

thanks

Phil

I bought one and had no luck with it. IIRC, I couldn't get consistent readings, and the plastic tube you stick in the tailpipe melted. If you've got 'bunsen blue' with Colortunes just lean the carbs a little and see if it pings.
 

steveg

Yoda
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I bought one and had no luck with it. IIRC, I couldn't get consistent readings, and the plastic tube you stick in the tailpipe melted. If you've got 'bunsen blue' with Colortunes just lean the carbs a little and see if it pings.

Mine runs best just on the yellow side of bunsen blue. Have a hill near my house and coming down it should just burble. Backfiring is too lean.

I have a switched K&N Air/fuel meter with an O2 sensor in each downpipe. This is mainly good for showing me if one of the carbs is acting differently from the other during acceleration mode.
 
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